In a hemodialysis treatment, the blood to be treated flows through the first chamber of a dialyzer, which is divided by a semipermeable membrane into a first chamber and a second chamber, whilst dialyzing fluid flows through the second chamber of the dialyzer. The hemodialysis treatment requires the balancing of fresh dialyzing fluid against used dialyzing fluid, in order to control the quantity of fluid fed to and removed from the patient. In hemodialysis, great demands are made on the balancing of the fluids.
In the case of acute dialysis for use in intensive care units, extracorporeal blood treatment apparatuses are used in which the fluids required for the blood treatment are made available in containers, in particular in bags. In the case of the known hemodiafiltration apparatuses for acute dialysis, dialyzing fluid and substituate are made available in bags, whilst filtrate is collected in a bag. To convey the fluids, use is made of peristaltic pumps, on the delivery accuracy of which great demands are made in order that the flow rate of dialyzing fluid, substituate and filtrate can be precisely adjusted.
The balancing of the fluids takes place in the case of the blood treatment apparatuses intended to be used in intensive care units by the fact that the weight of the bags filled with the fluids is monitored. The known blood treatment apparatuses comprise balances for this purpose. The flow rates for dialyzing fluid (dialysate), substituate and filtrate are preselected for the blood treatment, said flow rates being adjusted with the peristaltic pumps. However, since the actual delivery rates of peristaltic pumps, in particular roller pumps, diverge in practice from the setpoint delivery rates adopted in the drive circuit, incorrect balancing may occur. The exact balancing is achieved by the fact that the delivery rate of one or more pumps is changed in such a way that the difference between the weight reduction per unit of time of the dialysate bag and the substituate bag and the weight increase per unit of time of the filtrate bag corresponds to a preset value. If no fluid is to be removed from the patient, the pumps are regulated in such a way that the sum of the weight reduction of dialysate and substituate exactly corresponds to the weight increase of filtrate. Otherwise, a specific quantity of fluid is fed to or removed from the patient (ultrafiltration).
In the balancing, account must be taken if further fluids, for example heparin or citrate and calcium, are administered to the patient for the purpose of anticoagulation. The administration of further fluids can be taken into account by a manual adjustment carried out by the operator or automatically with the filtrate flow. For reasons of simplification, however, this will not be discussed further in the following description.
Only the measurement of the sum of the weight of the dialysate bag and substituate bag on the one hand and the weight of the filtrate bag on the other hand is in principle required for the balancing. Consequently, blood treatment apparatuses are known which comprise only two balances. Blood treatment apparatuses with three balances are however also known, wherein the dialysate bag and substituate bag are weighed with separate balances, so that the weight of each individual bag can be determined.
The control unit of the known blood treatment apparatuses provides a specific control range for the flow rates of the pumps, which in practice can amount to, for example, ±20% of the adjusted setpoint flow. If incorrect balancing is present, the incorrect balancing can be compensated for by changing the flow rate of one or more pumps within the control range. For example, in the case where the actual delivery rate of the dialysate pump is less than the setpoint delivery rate adopted in the drive circuit, the delivery rate of the dialysate pump can be increased accordingly. In order to compensate for the incorrect balancing, however, the required increase in the delivery rate of the dialysate pump must lie within the control range, i.e. for example within the control range of ±20% of the setpoint flow rate, in order to compensate for the incorrect balancing. The control cycle amounts to, for example, 1 second.
The known balances for weighing the bags are precision balances, which have to be checked on a regular basis. Known blood treatment apparatuses therefore provide cyclical tests of the balances which are carried out automatically. The cyclical balance tests are intended to avoid not only incorrect balancing due to errors with the balances, but also incorrect balancing caused by leakages in the hose system, which are revealed in the balance tests by an unpermitted drift of the measured values.
For a cyclical test of the balances, the pumps must be stopped in order to allow the balances to be checked. An attempt is therefore made to keep the number of cyclical tests as low as possible. The interval between successive balance tests must be selected in such a way that the incorrect balancing remaining undetected in the meantime cannot be greater than a specific value, for example 500 ml. In order to remain undetected in the meantime, a balance error or a leakage in the hose system must be so small that the balance error or the leakage can be compensated for by the permitted control range of, for example, ±20% of the adjusted setpoint flow, so that a balancing deviation cannot occur. The time interval between the balance tests is therefore dependent on the flow rate and the permitted control range. The time intervals between the cyclical balance tests are therefore also fixed in order to avoid the limiting value for incorrect balancing between two cyclical balance tests being exceeded. The smaller the permitted control range, therefore, the greater the interval between the cyclical balance tests can be selected.